1. Field of the Invention
The present invention relates to a method of starting operation of a solid polymer electrolyte fuel cell, which generates electrical energy by means of electrochemical reactions occurring between an oxygen-containing gas and a fuel gas. The solid polymer electrolyte fuel cell is formed by stacking a membrane electrode assembly and separators. The membrane electrode assembly includes a cathode and an anode as a pair of electrodes, with an electrolyte interposed between the cathode and the anode. The solid polymer electrolyte fuel cell has an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode, and a fuel gas flow field for supplying a fuel gas to the anode.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode in order to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators, so as to form a unit cell for generating electricity. In use, normally, a predetermined number of unit cells are stacked together to make up a fuel cell stack. The fuel cell stack is mounted in a vehicle, for example.
In the fuel cell, a fuel gas flow field is formed on a surface of one separator, which faces the anode, for supplying fuel gas to the anode, and an oxygen-containing gas flow field is formed on a surface of the other separator, which faces the cathode, for supplying oxygen-containing gas to the cathode. Further, a coolant flow field is formed between adjacent separators of the fuel cells for supplying a coolant into electrode areas of the separators.
In this type of fuel cell, water is produced during power generation. When power generation is stopped, the water, which was produced during power generation, tends to be retained on the downstream side of the oxygen-containing gas flow field and the fuel gas flow field. Further, when operation of the fuel cell is stopped, in a case where scavenging of the oxygen-containing gas flow field and the fuel gas flow field is performed using air, the cathode may become degraded undesirably, in particular, due to a high potential on the downstream side of the oxygen-containing gas flow field at a time when operation of the fuel cell is started.
In an attempt to address this problem, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2005-149838 is known. The fuel cell system includes a fuel cell for generating electrical energy using a fuel gas, which is supplied to a fuel gas flow field, and an oxygen-containing gas, which is supplied to an oxygen-containing gas flow field. The fuel cell system also includes a first fuel gas supply means for supplying fuel gas to the fuel gas flow field, an oxygen-containing gas supply means for supplying oxygen-containing gas to the oxygen-containing gas flow field, and a second fuel gas supply means for selectively supplying fuel gas to the oxygen-containing gas flow field.
When operation of the fuel cell system is started, supply of fuel gas to the oxygen-containing gas flow field is started before the fuel gas flows over at least the fuel gas flow field. Further, after the fuel gas starts to flow over at least the fuel gas flow field, switching is carried out in order to supply the oxygen-containing gas to the oxygen-containing gas flow field.
In this manner, gas that remains when the fuel cell system is started is utilized to suppress a corrosive reaction of carbon in the electrodes of the fuel cell. According to the disclosure of Japanese Laid-Open Patent Publication No. 2005-149838, degradation of the cathode (oxygen-containing gas electrode), which may occur when the starting operation is carried out, is suppressed effectively without requiring the system to be large in size.
In Japanese Laid-Open Patent Publication No. 2005-149838, fuel gas is supplied over the entire fuel gas flow field, as well as over the entire oxygen-containing gas flow field when operation of the fuel cell system is started. Therefore, the amount of wastefully consumed fuel gas is large, which is uneconomical. Further, in the cathode system, catalytic combustion tends to occur due to mixing of hydrogen and oxygen, resulting in degradation of the electrode.